1. Field of the Invention
The present invention relates to motor control devices that control a motor and to compressors.
2. Description of Related Art
A load factor of a motor often entails periodic load variations. A compressor used in an air conditioner or the like is an example of a product including such a load factor. In a hermetic type compressor used in an air conditioner, a change in refrigerant gas pressure during cycles of suction, compression, and discharge is known to affect a load torque. It is also known that the load torque associated with the refrigerant gas pressure varies in synchronism with the rotation of the compressor, resulting in periodic variations in rotation speed of the compressor. Such periodic variations in rotation speed of the compressor produce vibration in the compressor itself, and also cause noise.
To solve the problems such as vibration and noise caused by variations in load torque, various methods have been proposed. For example, there has been proposed a conventional method adopting torque control that estimates a variation component of a torque produced by a load device and compensates for the variation component thus estimated, wherein a current component for compensating for the variation component is limited by a limit value.
For example, in another conventional method, a torque variation component is extracted based on, for example, the deviation of the estimated motor speed from the specified motor speed value, and the extracted torque variation component is used to reduce vibration.
In addition to reducing vibration and noise of a device, reducing power consumption in a system for driving a motor is an important problem to be solved.